1. Field of the Invention
The present invention relates to a mobile communication system, and a method for operating the same. More particularly, the present invention relates to a mobile communication system using a multiplex zone configuration.
2. Description of the Related Art
In order to flexibly cope with a difference in topography or traffic, a multiplex zone configuration is often used for a mobile communication system. In the mobile communication system employing a multiplex zone configuration, the congestion in a communication channel is desirably reduced. Furthermore, the call quality between a base station and a mobile terminal is desirably kept at a good condition.
Such a mobile communication system is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 9-130845). In the conventional mobile communication system the service area is covered with two kinds of zones, that is, large zones and small zones. The large and small zones overlap with each other. One of the large zones or the small zones is selected for the communication of a mobile terminal. The selection is executed based on the kind of the mobile terminal.
The known mobile communication system, as depicted in FIG. 1, includes a base station controller 101, base stations 102 to 107 and mobile terminals 108, 109, as shown in FIG. 1. The base stations 102 to 107 can communicate with the mobile terminals within the radio zones 102a to 107a, respectively. Here, the radio zones 102a, 103a are the large zones, and the radio zones 104a to 107a are the small zones.
The base station controller 101 sends a control signal indicating whether the radio zone in which the mobile terminal exists is the large zone or the small zone, to the mobile terminals 108, 109 within the radio zones 102a to 107a, through the base stations 102 to 107. The mobile terminals 108, 109 refer to the control signal, and judges whether the mobile terminals 108, 109 exist within the large zone or the small zone.
Which one of the large zone and the small zone is used as a priority zone is defined based on the kind of the mobile terminals 108, 109. Each of the mobile terminals 108, 109 makes a call by preferentially using the radio zone, which is the priority zone, of the large zone and the small zone.
Also, another mobile communication system employing a multiplex zone configuration is disclosed in Japanese Laid Open Patent Application (Jp-A-Heisei 5-14963). In the other mobile communication system, a plurality of small zones are accommodated in a large zone. A hand-over between two of the small zones is carried out based on the receive levels of the channels between a mobile station and the two of the small zones.
In the mobile communication system employing the multiplex zone configuration, the congestion of communication channels between mobile terminals and base stations is desirably reduced. Especially, the congestion of the communication channels is desired to be reduced when traffic of the communication channels between the mobile terminals and the base stations is increased.
Also, in the mobile communication system employing the multiplex zone configuration, it is desirable that a hand-over is not frequently induced even if a mobile terminal moves at a high speed.
Moreover, in the mobile communication system, it is desirable that an unbalance is not easily induced between a load on a base station communicating with a mobile terminal within the large zone and a load on a base station communicating with a mobile terminal within the small zone.